Digital imaging systems have created a revolution in photography and cameras. A digital camera is similar to a film camera except that the film is replaced with an electronic sensor. The sensor is comprised of an array of photo detectors that change the photons that strike them into electrons providing a signal at each pixel proportional to the number of photons, or the amount of light at each pixel. Presently, most consumer digital cameras employ Charge Coupled Device (CCD) or Complementary Metal Oxide Semiconductor (CMOS) sensors. To facilitate the collection of light, many of the sensors employ a small lens-like structure covering each pixel, which is called a microlens. These microlenses are typically made by forming a layer of photoresist that is placed upon the pixel frame.
The image sensors used in digital imaging are inherently monochrome devices, having no color discriminating ability associated with each detector. For this reason, the sensors typically employ a color filter array (CFA) inserted between the microlens and each active portion of the pixel structure. Typically the CFA is constructed to assign a single color to each pixel. Digital camera manufacturers often chose among a variety of CFA architectures, usually based on different combinations of primary colors (red, green, blue) or complementary colors (cyan, magenta, yellow). Regardless of the particular CFA used, the overall aim is to transfer only a single color of interest, so that each pixel only sees on color wavelength band.
One of the most popular CFA patterns is called a Bayer pattern, which places red, green and blue filters over the pixels in a checkerboard pattern that has twice the number of green squares as red or blue. The theory behind the Bayer pattern is that the human eye is more sensitive to wavelengths of light in the green region than wavelengths representing red and blue. Therefore, doubling the number of green pixels provides greater perceived luminance information and detail, while providing a natural color representation for the human eye.
When subjected to light, the image sensor converts incident photons to electrons. The conversion enables analog electronic circuitry to process the image “seen” by the sensor array. The electrons gathered by these sensors are stored in small capacitors that are read out as a series of varying voltages, which are proportional to the image brightness. An Analog to Digital Converter (ADC) conditions these voltages for processing by a computer within the camera. The data is then processed to form a picture of the image “seen” by the sensor array.
As digital imaging becomes more prevalent, industry is striving to develop images and video with better resolution and color accuracy, while also striving to reduce cost and complexity in digital cameras. Typically, there is a tradeoff in digital cameras among costs, size and quality of optics. Low cost and relatively small size optics are required for some applications, such digital cameras that are included in wireless phones. In these applications design requirements may lead to spatial variations in image attributes and quality over the image. These variations can lead to unacceptable image quality.
Furthermore, the image sensors themselves are prone to defects which can produce visible and undesirable imaging errors. For example, some pixel errors can result in dark spots in bright areas, and/or bright spots in dark errors. These defects can be highly unexpected and thus draw the viewing eye to them by fundamental sensory mechanism. The result again can lead to unacceptable image quality.